ADSORPTION INDUCED ANISOTROPY IN THE ELECTROREFLECTANCE OF SILVER

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ADSORPTION INDUCED ANISOTROPY IN THE

ELECTROREFLECTANCE OF SILVER

T. Furtak

To cite this version:

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JOURNAL DE PHYSIQUE Colloque C5, supplkment au no 11, Tome 38, Nouembre 1977, page C5-233

ADSORPTION INDUCED ANISOTROPY

IN THE ELECTROREFLECTANCE OF SILVER

T. E. FURTAK

Ames Laboratory-ERDA and Department of Physics, Iowa State University, Ames, Iowa 5001 1, U. S. A.

Rbumk.

-

Nous fournissons la premikre preuve document& d'anisotropie induite par adsorption dans la reflexion modulee en champ Clectrique (Clectrorkflexion). Dans I'argent l'effet est dQ

a une couche de rkaction produite sur une surface a double symCtrje d'un monocristal pendant son electropolissage dans une solution de cyanure d'argent. Pour une lumiere ?incidence normale i

polarisee parallklement a I'axe cristallin [oo~], la structure de l'electroreflexion entre 290 nm et 400 nrn s'interprete a I'aide du modtle de I'Clectron libre, mais elle est caracterisk par une perturbation de liaison lorsque la lumikre est polarisk parallklement h I'axe [liO]. Ce comportement est discutk en admettant une liaison probable entre les ions de cyanure et le rkseau de I'argent. Nous proposons aussi des extensions possibles et des applications de 1'electrorCflexion anisotropique.

Abstract. - The first documented evidence for adsorption induced anisotropy in electric field modulated reflectance (electroreflectance) is presented. The effect, in silver, is caused by a reaction product layer which is produced on a single crystal surface, of two-fold symmetry, during electropolishing in an argentocyanide solution. The electroreflectance structure from 290 nm to 400 nm appears freeelectron-like when the normally incident light is polarized parallel to the [00i] crystal axis, but displays evidence of bonding perturbation when the light is polarized parallel to the [liO] axis. This behaviour is discussed in terms of the probable bonding of cyanide ions to the silver lattice. Extensions and applications of anisotropic electroreflectance are also suggested.

1. Introduction. - Electric field modulated reflectance, induced by an increment in voltage applied to a metal-electrolyte interface, is often difficult to interpret. This is because the normalized reflectance change, ARIR

=

A In R, is in many cases [I-41 caused by a change in the surface concentration of electroactive adsorbates (a), which interact with the photons, in addition to a field-induced (8) change in the metal

surface. True electroreflectance A, In R is not compli- cated by optical effects due to a modulated adsorbate coverage. However, since the region of electric field modulation is localized at the surface of a metal, electroreflectance should be very sensitive to changes in the metal surface electron structure which are caused by a metal-adsorbate bond.

To increase the capability of this technique, it is useful to identify some feature of A, In R which is unambiguously connected with the optical response of the metal. This characteristic appears in the electroreflectance of single crystal 110 Ag. In previous experiments [5, 61 it was shown that A, In R displays a polarization

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anisotropy Az(A, In R), even at normal incidence. The effect is caused by a modulation of the excitation probability which describes the interaction of photons with electrons which are under the influence of the spatially periodic lattice potential, as well as the modulating field. This phenomenon will be most useful for studying the metal's role in adsorption if the adsorbate itself does not strongly modify

the photon beam, and the adsorbate coverage does not strongly depend on the applied voltage. When these conditions prevail, the anisotropy in electroreflectance, Ae(A, In R), and its change upon adsorption, A,(Ae(A, In R)), can be a very sensitive probe of the contribution of the metal surface atoms to bonding. Such is the case for the reaction product layer which strongly adheres to silver as a result of eIectropoIishing in argentocyanide solution. This report describes how the anisotropy in the normal incidence electroreflectance from 290 nm to 400 nm on 110 Agchanges in the presence of this layer. High vacuum techniques are described which were used t o identify the major componcnt of the adsorbate as CN-. This information has led to proposed metal-cyanide bonding models which are consistent with the optical data.

It may seem surprising that a perturbation applied normal to the surface (as is the case with the double- layer electric field) can modify the metal so that it responds anisotropically to normally incident photons. In the absence of an applied field, a cubic mate- rial (as is silver) responds isotropically to a photon probe. Figure 1 illustrates how thc symmetry is lowered from the 0, point group by applying an electric field along the three low index directions. The dielectric function becomes a tensor, which is uniaxial when the field is applied along

<

100

>

and

<

I l l

>,

but biaxial with the field along

<

110

>.

This means the contribution to electroreflectance which probes

16

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C5-234 T. E. FURTAK

"3

< l o o > < I l l > < ! l o >

FOUR-FOLD THREE-FOLD TWO- FOLD

FIG. 1.

-

Symmetry characteristics of a cubic crystal with an applied field along the

<

100

>, <

111

>,

and

<

110

>

axes. The point groups are C4v, Csv, and Czv respectively. The forms of the dielectric tensors, referred to the numbered axes, are also shown. The optical response is biaxial on the 110 surface.

the symmetry environment of the metal surface of a cubic crystal, under the latter conditions, will, in general, depend on the orientation of the polarization vector of the light.

2. Experimental. - This experiment was performed in two separate electrochemical cells. In this way, the adsorption step was isolated from the voltage modulation causing the electroreflectance. The polishing cell was used to create an optically smooth surface on the exposed face of the teflon encapsulated sample, and to deposit an unknown amount of adsorbate. This occurred as the result of silver decomposition, under potentiostatic control (0.25 V versus SCE), in the air saturated solution of 35 g/l AgCN, 37 g/l KCN, and 38 g/l K2C03 171. After a brief rinse in triple-distilled water, the surface appeared to be clean and topographically perfect. In this condition, the sample was transported, in air, to a second chamber filled with a nitrogen saturated solution of

4

M H,SO,, where the electroreflectance was recorded. The applied voltage was maintained at - 0.2 V versus SCE with a superimposed 70 mV rms, 100 Hz sine wave. A monoenergetic (bandwidth of 3.3 nm), polarized photon beam was introduced at an incident angle of 100 through the quartz wall of the cell. The reflected beam intensity, detected by a photomultiplier, contai- ned an alternating component, proportional to A , R which was monitored by a lock-in amplifier, and a background level, proportional to

R,

which was held constant by adjusting the photomultiplier gain.

The spectral behaviour of the electroreflectance varies with the bias level but, providing the voltage excursions are limited within OV to

-

0.4 V, the spectra are reproducible upon returning to

-

0.2 V. When these same experiments were performed in 1 M KOH, the bias voltage range over which this effect persisted was extended in correspondence with the negative

extension of the equilibrium voltage for hydrogen evolution. This is the major evidence that the adsorbate is strongly bound and that desorption is not voltage driven. If it were, then, as the adsorbate went into solution, it would drift away from the sample and be unavailable for re-adsorption. This experiment clearly measures A , In R not A , In R.

It is possible to remove the adsorbate by chemical action. This technique, which exploits the reducing power of atomic hydrogen electrochemically gene- rated on the electrode, has previously been used to obtain an uncontaminated surface for optical [ 5 , 61

and electrochemical [8] measurements on clean silver. It was possible to generate hydrogen with adequate efficiency in

3

M H2S04 by maintaining the applied voltage at

-

0.5 V versus SCE.

The experiment was performed by introducing the adsorbate covered silver sample into the measuring cell and immediately recording A, In R versus photon wavelength for two perpendicular orientations of the optical polarization vector. The crystallographic directions

<

110

>

and

<

001

>,

used in this discussion to identify the polarization, can be associated with directions in the unit cube in the last frame of figure 1 by vectors 1 and 2 respectively. The incident beam, surface normal

<

110

>,

and applied field are, in this same figure, parallel to vector 3. Short hydrogen evolution treatments were alternated with succeeding electroreflectance records. The result was a picture of how the sample gradually progressed to its clean state.

3 . Results. - The spectra of A , In R versus photon wavelength appear in figure 2 for optical polarization parallel to

<

001

>

and figure 3 for polarization parallel to

<

IT0

>.

In each of these figures, curve A was recorded first with the highest coverage of adsorbate. Curves B through H were recorded with a progres- sively smaller coverage of adsorbate brought about by in sitzr hydrogen treatments. Hydrogen was evolved for 20 seconds before each of scans B through D.

4 0 0 3 5 0 300

PHOTON WAVELENGTH (nm)

FIG. 2. - Electroreflectance on 110 Ag with the optical polarization parallel to 001. Curves H through A were recorded with

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ADSORPTION INDUCED ANISOTROPY IN THE ELECTROREFLECTANCE OF SILVER C5-235

Longer treatments were employed before the other Negative secondary ion mass spectroscopy (SIMS) scans. The total treatment time before scan H was performed in vacuum is useful for identifying the

530 seconds, beyond which no change in A, In

R

negative ionic component of the extreme surface of a could be discernecl. sample. Two electropolished silver crystals, one of which had been subjected to a prolonged hydrogen evolution treatment, were analyzed in our laboratory ~ i o with this technique. Figure 5 shows the comparison

3 . 8 4 e V

-

2

4 0 0 3 5 0 3 0 0

PHOTON WAVELENGTH (nm)

FIG. 3.

-

Electroreflectance under the same conditions as in

figure 2, except the optical polarization was parallel to 170.

To more completely identify the adsorbate, let us first compare figures 2 and 3 to electroreflectance on

110 Ag recorded under a variety of conditions, as shown in figure 4 15, 61. Note that spectra b were taken with an electropolished and hydrogen cleaned sample, whereas spectra a were taken with a sample which was chemically polished in a hydrogen perox- ide-ammonium hydroxide etch. The dominant features in this figure as well as figures 2 and 3 are essentially the same for a given polarization orientation. From this comparison it is clear that the adsorbate is a result of the electropolishing and that curves H in figures 2 and 3 are characteristic of clean silver electroreflec- tance.

I I I I I I I I I I I

I

3 0 3 2 3 4 3.6 3 8 4 0 4.2

PHOTON ENERGY (eV)

FIG. 4. - Ebctroreflectance on 110 Ag under the same optical conditions as figures 2 and 3. (a) Chemically polished surface, recorded in 1 MHC104 at 0 V versus SCE using 54 mV rms,

1 kHz modulation. (b) Electropolished and hydrogen cleaned

surface, recorded in 1 M KOH at - 0.35 V versus SCE using

35 mV rms, 1 kHz modulation. This shows that the clean surface electroreflectance is not an artifact of the measuring

technique.

ELECTROPOLISHED SILVER

lH-

₁

HYDROGEN TREATED 1 H- NOT HYDROGEN TREATED

FIG. 5. - Secondary ion mass spectra of two electropolished silver crystals. Only one was subjected to the hydrogen reduc- tion treatment. The untreated sample shows a much larger

concentration of CN-.

of the mass spectral yield on each of these surfaces. The lower intensity peaks are due mainly to hydro- carbon (note the high yield of H-) and oxygen contamination. The most obvious difference between the two is the presence of a high of negative ions of mass 26 on the untreated surface which is considerably smaller on the hydrogen reduced surface. Although some of this may be attributed to C,Hi, the majority is almost certainly CN-. To complete the surface analysis, Auger electron spectroscopy (AES) as a function of depth into the sample surface (obtained by argon ion bombardment) was recorded in the same vacuum chamber on the same samples. The results, in figure 6,

SPUTTER TIME (rn1n.1

FIG. 6 .

-

Auger electron spectra versus argon ion sputter time (proportional to depth below the surface) for the two samples described in figure 6. Nitrogen, from the CN-, is present only on the untreated sample. Both surfaces are virtually free from other

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C5-236 T. E. FURTAK

show nitrogen on the untreated sample which is with the crystal lattice as was verified by rotating the clearly located only at the surface, whereas no nitro- sample about the surface normal. Since the cyanide gen was seen on the treated sample. These results coverage was not voltage dependent within the modu- unambiguously identify the adsorbate as CN-. The lation limits, there was no complication from a vol- discussion from this point on will center on this con- tage driven A , In R. This automatically excludes any clusion. It was clear from the vacuum surface analyses interpretation based on surface concentration modu- that both samples were extraordinarily clean. Com- lation [4]. Silver cyanide is a wide band-gap insulator parison between the N and Ag Auger intensities leads which does not appreciably absorb photons below to the conclusion that, at least in vacuum, the CN- 4.6 eV [13]. Therefore, direct, field driven modulation coverage is much less than a monolayer. It remains to of the optical properties of a partial coverage of AgCN be determined whether some fraction of the original must also be excluded. The most probable explana- C N - leaves as a result of its being subjected to the tion is that the CN-, which uses some of the silver s-p high vacuum of the surface analysis chamber. states to form a strong chemical bond, causes a distortion of the photon interaction with those states. 4. Discussion.

-

Curves H in figures 2 and 3 _{What we see in A , In}

R

then, is the effect of cyanide (clean silver) have been described before [5, 61. The _bonding _{on the silver.}

polarization insensitive features have been explained _{1, figure 2, the basic form}_of_{the free-e[ectron-like} by J. D. E. Mclntyre 191. His theory relies on a modu- _{electroreflectance is maintained as CN- is added. The} lation of the free-electron density in the metal surface. _{structure merely shifts to higher energy with increas-} Employing the three-phase optical model he develop- _{ing coverage. By contrast, figure 3 shows a pronounced} ed with D. E- Aspnes [lo], McInt~re's theory SUCces- _{sign change in}_A,_In

_R

_{in the presence of CN-. The} sfully predicts the sign, general shape, and location _{location of structure is also seen to shift to higher} on the photon energy axis of the major structure in _{energy. The McIntyre-Aspnes description of inter-}

A, In R. This is not the entire story, however. T. E. Fur- _{facial modulation spectroscopy predicts that structure} tak and D. W. Lynch [5] later demonstrated that it is _in_A_In_~i_{will occur near the dip in the imaginary part} not valid to treat the surface electrons as completely _{of the inverse of the substrate dielectric function [9].} free. Their results suggest that field-induced indirect _{T G ~}_{is generally applicable and independent}_of_the interband transitions play a role in determining _{mechanism. ,rhis dip will shift to higher}

A, In R. ~ h e s e transitions, from a P-like band a t the _{photon energy if the energy for transitions giving rise} Fermi surface to an d i k e band (P _s), _{to structure in the substrate dielectric function is} follow the crystal lattice symmetry and thus contri- _{increased. This would occur if the substrate Fermi level} bute the anisotropy On 11° Ag as by were to be depressed in the surface region of a result

the difference in curve H in figures 2 and 3. Transitions of an electron deficiency. ~h~ shift of structure in which occur above 4.1 eV [ l l ] from the silver d-states _A, _{to higher energy with adsorption would then} to the p-band at the Fermi surface are not modulated be attributed to an adsorption induced net positive by this mechanism since the lower energy final states charge on the silver surface.

in the p-band are already occupied. It is the distortion _J _Z _{7 illustrates the spatial configuration and}_~ _~ _~ _~ _~ of p + transitions cyanide bonding which gives energy ordering of molecular orbitals associated with

rise to the change in the anisotropy of A , In R. _CN-_1141._{By analogy with CO on metal surfaces [15]} These data might be analyzed by Kramers-Kronig _{and A ~ C N}_{[14] it can be proposed that the}

integration to ~ventually get the change in an effective ₅_a_{levels nearest the carbon donate electron density} complex dielectric function of the silver surface [12]. _to_the _s-p_{states. hi^ is the primary} _cause_of_the This inversion is quite sensitive to assumptions about _{bonding perturbation}_seen _in_{A , in}

_R.

_{~h~ shift of} the optical structure of the interfacial region. Since the _{structure to higher}_{energy must then be caused by} cyanide coverage is thought to be less than one mono- _{back donation of silver d-electrons into the antibond-} layer, the meaning of an effective surface dielectric _{ing cyanide 2}_n*_levels.TO produce a net positive function would be difficult to interpret and may, in

fact bring out structure which was merely an artifact

2 r a of the inversion. Although A, In

R

does not directly

show elementary absorption, it does represent the

I

5u ( u 2 p ) 2

true experimental situation. A more detailed analysis

I r ( r 2 p ) 4 must be postponed until more sophisticated models

are developed. E 4 u 4 ( ~ ' 2 s ) ~

The remainder of this discussion will be concerned 3~ ( 0 2 s ) ~ with specific features of A, In R along with tentative

explanations. It must be re-emphasized that the ani-

FIG. 7. - Schematic representation of the spatial orientation o .

'Otr0py is due an optical reflection since

molecular orbitals in CN-. Filled levels are shaded. Also shown

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ADSORPTION INDUCED ANISOTROPY I N THE ELECTROREFLECTANCE O F SILVER C5-237

charge on the metal surface as seen in the optical data, this back-donation must over-compensate the o bond. An alternate explanation is based on a weak o bond without back-donation, leaving a negative charge on the CN- ion. This, however, would result in a much weaker bond than is experimentally observed and is inconsistent with the covalent nature of the AgCN molecule.

It is more difficult to identify the microscopic mechanism for the observed change in the anisotropy. An adsorbate induced silver lattice surface reconstruction would modify the s and p energy states. Normally less pronounced anisotropy caused by field induced indirect interband transitions between these states might then be expected to change character. It is also possible that an adsorbate-adsorbate interaction causes a perturbation of the s-p electron distribution along certain directions. This is supported by the fact that the anisotropy modification is most pronounced with the light polarized parallel to

<

1i0

>,

the direction along which the surface lattice spacing is the smallest. It is difficult to understand, however, why this would be important if the cyanide coverage, as suggested, is much less than a monolayer.

Even in the absence of complete understanding, it is clear that anisotropic electroreflectance is a power- ful diagnostic. Already it can be concluded that the argentocyanide electropolish involves an intermediate step, most likely Ag

+

CN- -t AgCN

+

e-, [16] which causes a strongly adherent thin coverage of CN-, even in a solution with a large excess of free CN-. This layer must be removed before clean silver surface studies can be performed.

It should be possible to study a variety of adsorption problems exploiting the anisotropic behavior of electroreflectance on 110 Ag. Halide ions and metal monolayers would be suitable as optical data for these systems already exists [I-41. The understanding of cyanide bonding would be aided by studies using ethylcyanide and ethylisocyanide, materials which present lone pairs of electrons projecting from N and C respectively. Rhodium, with its unfilled d-states, which might directly participate in a bonding, would make an interesting complementary substrate for this work. As in the study of cyanide on silver, it would be ideal if the adsorption could be separated from the electroreflectance, but this may not be unambiguously possible.

This report has described the use of adsorption induced changes in the anisotropy of normal incidence electroreflectance on single crystal silver to empha- size bonding perturbation in the metal surface. This technique is at least as sensitive as AES and SIMS to the presence of cyanide and has the added feature that it is blind to weakly bound airborne contamination. In addition, it is particularly suitable for use as an

in situ diagnostic where a separate vacuum analysis is impossible or underisable. In the future, lt should prove to be a valuable application of optical techniques to electrode characterization.

Acknowledgments.

-

My thanks go to A. Bevolo of Ames Lab who performed the SIMS and AES analyses. This work was supported by the U. S. Energy Research and Development Administration, Division of Physical Research.

References

[I] KOLB, D. M., LEUTLOFP, D. and PRZASNYSKI, N., Surf: Sci. 47 (1975) 622.

[2] BEWICK, A. and THOMAS, B., J. Electroanal. Chem. 65 (1975) 911.

[3] TAKAMURA, T., WATANABE, F. and TAKAMURA, K., Electro-

chim. Acta. 19 (1974) 933.

[4] LAZORENKO-MANEVICH, R. M., BRIK, E. B. and KOLO-

TYRKM, Y. M., Electrochim. Acta 22 (1977) 151. [5] FURTAK, T. E. and LYNCH, D. W., Phys. Rev. Lett. 35 (1975)

960.

[6] FURTAK, T. E. and LYNCH, D. W., J. Electroanal. Chem. 79

(1977) 1.

[7] SETTY, T. H. V., Indian J. Chem. 5 (1967) 5.

[8] VALETTE, G. and HAMELIN, A., J. Electroanal. Chem. 45

(1973) 301.

[9] MCINTYRE, J. D. E., in Advances in Electrochemistry and

ElectrochemicalEngineering, Delahay, P. and Tobias, C., editors (Wiley-Interscience, New Y ork) 1973, 61. [lo] MCINTYRE, J. D. E. and ASPNES, D. E., Surf: Sci. 24 (1971)

417.

[ l l ] ROSEI, R., CULP, C. H. and WEAVER, J. H., Phys. Rev. B 10 (1975) 484.

[12] PLIETH, W. J. and NAEGELE, K., Surf. Sci. 50 (1975) 53. [13] GALLAIS, M. F., C. R. Hebd. Sgan. Acad. Sci. 214 (1942) 552. 1141 GRIFFITH, W. P., Q. Rev. 16 (1962) 188.

[I51 BRADSHAW, A. M. and PRITCHARD, J., Proc. R. SOC. A 316

(1970) 169.

[16] LEBEDEV, A. N., GUBAILOVSKII, V. V. and KA~ovsKrr, I. A.,

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T. E. FURTAK

JSSION

W. HANSEN. - While the static electric field may penetrate only an angstrom or so into your sample, this changed region is sampled by electrons from several hundred angstroms depth whose properties are thereby affected. The radiation, also penetrates a few hundred angstroms. It seems therefore that an anisotropic response can result from free electron interaction with the anisotropic surface. Would you comment ?

T. FURTAK. - The penetration of a non-zero static electric field certainly is deeper than the Thomas-Fermi screening lengh. This parameter is only a rough measure of the extent of a continuously varying field which may exist at an appreciable magnitude, several angstroms below the surface.

The mecanism you propose is certainly possible. An electron with a mean free path of tens of angstroms could interact with both the periodic lattice potential, which is the origin of the anisotropy, and the app2ied electric field in the immediate surface region, which gives rise to the electromodulation.

M. FROMENT.

-

VOS rtsultats d'ClectrorCflectance peuvent effectivement s'interprbter par une anisotropie d'adsorption des cyanures. Des dtptits Clectrolytiques effectuts ulttrieurement sur vos surfaces pourraient Cventuellement rCvCler ces sites d'adsorption, car ces derniers bloquent la croissance.

T. FURTAK. - This would be a good way to solve the question of the quantitative nature of cyanide surface coverage.

I. EPELBOIN. - On connait mieux le polissage Clec- trolytique des mttaux dans la solution de Saquet (brevet 1929) B base de C104 que dans les solutions Spitalsky (brevet 1905) a base de CN- que vous utilisez pour l'argent. Cependant, les oscillations du courant caractkristiques du polissage de l'argent en prtsence des ions CN- laissent supposer 1'Ctablissement des ttats stationnaires multiples dus probablement comme dans le cas de la passivation du fer en milieu acide (voir par exemple l'article publit par mon laboratoire en

1975 dans un NO spCc. de Z. fur Phys. Chem. N. F. (Francfort) consacrC

A

la mimoire de ~ e t t e r ) B un couplage entre des processus de transfert de charge et transport de matitre. Se me demande si votre mCthode d'tlectrortflectance est sensible B la dCtection des films passivants qui se forment au cours du polissage de l'argent en prtsence des ions CN- ?

T. FURTAK. - It was important in this study to exclude the possibility that the modulating field was also modulating the coverage of adsorbate. It would indeed be possible to observe a passive layer if in its bonding to the silver, the silver lattice was perturbed in a different way than with cyanide bonding, thus leading to a distinctive modification of the anisotropy. In the case where the silver is actively decomposing however a modulation of field would, also lead to coverage and rate modulation in which case the electroreflectance would be much more difficult to interpret.

S. GOTTESFELD.

-

In relation to the nature of the layer formed on a metal during electropolishing, it may be noticed that a previous optical in situ measurement (Novak, Reddy, Wroblowa). On copper during electropolishing showed a thick surface layer during the process.

T. FURTAK. - This is quite right however for silver which has been polished in concentrated argentocyanide solution, and then removed from that solution, the only strongly bound reaction product is that involving the cyanide ion at relatively low coverage. M. THEYE. - Quels sont, dYapr&s vous, les Ctats Clectroniques liCs participant aux phCnom6nes d'klec- trortflectance dans Ag ? Pensez-vous 2i des Ctats de volume (et dans ce cas lesquels) ou k des Ctats de surface ?